You will need a large clear jar with a lid that can be sealed, a second container of some type, some Chlorine bleach, some Iodine, and two light colored sponges. Cut one sponge in the shape of a heart. Cut the other in the shape of the cross. Both need to be of a size that will fit in the jar with a lid.

Fill the jar with a lid about half full of water, and the other container with a very strong solution of bleach and water (half and half will do). Set the cross in the bleach solution as you begin your lesson (not too long before, it could disolve the sponge).

For the lesson, show the children the heart shaped sponge. Talk about sin and put some drops of iodine on the sponge to represent those sins. You can talk about how impossible it is to remove those stains by yourself. Even dip the heart shaped sponge in the jar of water and show it is still dirty. Then, put the heart shaped sponge in the jar of water and leave it. Now take the cross out of the bleach solution, keeping as much of the solution in the sponge as possible. Tell the children about the power of the cross to cleanse the heart from sin. Put the cross in the jar with water and the stained heart, seal it with the lid, and shake it up a bit as you talk some more about the cross. Then, open the lid and take the heart out. It will be clean.

You should experiment with this lesson once before you actually give it to be sure you have a strong enough bleach solution to clear the iodine from the heart shaped sponge. Be sure to rinse the heart shaped sponge very clean with clear water so the iodine stains will remain when you do the real lesson.

Here is a YouTube video of this process being used in the context of an alternative worship event: